The present study was designed to test a group-based format of dynamic assessment (G-DA) in the context of writing over a time span of twelve weeks of instruction. A cohort of 60 students took a homogeneity test and based on the results, 44 students were selected to participate forming the two groups of experimental (N=22) and control (N=22). The study benefitted from a mixed methodology design comprising both quantitative and qualitative data collection and analysis. The experimental group underwent G-DA instruction for a time span of 12 weeks and received prompts, hints and scaffolding during all stages of writing including topic selection, idea generation and revising while the control group was deprived of dialogic negotiation and interaction. The results of quantitative data analysis of pretest and posttest scores using independent and paired samples t-tests revealed the outperformance of the experimental group over the control group. The microgenetic analysis showed that the G-DA instructions could diagnose quite vividly the learners' sources of writing difficulties and help promote the abilities which are in the state of maturation. It was also found that the G-DA interactions could set the ground for creating a state of intersubjectivity and positive interdependence among the more and less proficient learners in the course of which they could trial their legitimate peripheral participation. The G-DA interactions had the function of moving the entire class forward in its ZPD while co-constructing ZPDs with individual learners within the social microcosm of the classroom context. On implication side, it is argued that the G-DA serves as a precise, teacher/learner-friendly and, thus, ethical procedure for the assessment of learners' writing abilities.